The present invention relates to casting drums for use on continuous casting machines and, more particularly, to a segmental casting drum that is operably associated with a pressure control system for evacuating the mold cavity prior to casting and subsequently pressurizing the mold cavity to eject the cast product therefrom.
A large percentage of the battery grids used in commercially-available lead-acid batteries are currently manufactured by a continuous casting (i.e., "con-cast") process. Traditional continuous casting machines include a rotary drum having a patterned mold cavity (i.e. a reticulated grid pattern) engraved into its outer peripheral surface, and a stationary shoe having an arcuate surface which overlays a limited portion of the mold cavity. The molten lead alloy is discharged through an orifice slot in the shoe such that it is directed into the mold cavity as the casting drum rotates past the shoe. Due to rapid solidification of the molten lead alloy, a continuous grid strip is formed and stripped from the drum upon rotation past the shoe. One example of a conventional continuous casting machine and the processing parameters associated therewith is disclosed in U.S. Pat. No. 4,349,067 issued to Wirtz, et al.
From studying the nature of grid defects attributable to con-cast processing, the most prevalent defects can be generally classified as either "metallurgical" or "mechanical". Metallurgical casting defects typically relate to improper grain size, equiaxed grains and/or grain boundaries which are primarily attributable to non-uniform solidification temperatures. Conversely, mechanical casting defects relate to cold knits, grid inclusions caused by dross (i.e., lead oxide), and voids caused by air entrapment or gas expansion in the mold cavity between the closely mating surfaces of the drum and shoe. Thus, it would be desirable to vent the mold cavity prior to delivery of the molten lead for purging trapped air and gas therefrom. However, unless the mating surfaces between the drum and shoe are maintained in close sliding engagement, the molten lead alloy will leak (i.e., flash) therebetween. Maintenance of such a close sliding contact is critical, yet difficult to control due to variations in thermal expansion of the casting drum across its entire width as molten lead alloy is directed into the mold cavity. Accordingly, it has been proposed heretobefore to provide a system for maintaining the drum temperature (i.e., a cooling system) during operation of the con-cast machine so as to eliminate or substantially minimize flash due to distortion of the casting drum. One example of a cooling system for a casting drum used with a continuous casting machine is disclosed in U.S. Pat. No. 4,489,772 to Wirtz, et al.
Despite the improvements presented by the last-mentioned Wirtz patent, the con-cast process still suffers from several drawbacks which significantly limit its production capacity as well as product quality and cost. In particular, to avoid the above-noted variations in thermal expansion of the casting drum, the size (i.e., width and diameter) of casting drums has heretobefore been limited which, in turn, limits the number of grid strips that can be concurrently cast (i.e., "side-by-side") on a single drum. Moreover, due to the high precision required for machining (i.e., engraving) the reticulated grid pattern into the outer peripheral surface of the drum, fabrication is expensive and defects commonly result in scrapping of the drum. Finally, conventional casting drums are not provided with any mechanism for removing entrapped air or gasses from within the mold cavity which are known to result in voids and inclusions in the grid.